A protester who disrupted this year's Boat Race by swimming into the path of the crews was today jailed for six months.

Trenton Oldfield, of Myrdle Street, east London, who was also ordered to pay £750 costs, was watched by millions of television viewers as he halted the annual race on the Thames between Oxford and Cambridge universities on April 7.

He was found guilty at London's Isleworth Crown Court last month of causing a public nuisance and returned to the same court today to be sentenced.

Oldfield, supported in court by a throng of well-wishers, smirked as Judge Anne Molyneux passed sentence. But he looked surprised as the jail term was handed down.

The judge said Oldfield had acted dangerously, disproportionately, had not shown what he was actually protesting against, and displayed prejudice in sabotaging the event which Oldfield regarded as elitist.

Judge Molyneux said Oldfield ruined the race for everyone: "You caused delay and disruption to it and to the members of the public who had gone to watch it and to enjoy the spectacle of top athletes competing.

"The rowers had trained for many months. You had no regard for the sacrifices they had made or for their rigorous training when you swam into their paths."

Adding that Oldfield's actions had endangered his life and those of others, the judge said: "Every individual and group of society is entitled to respect. It is a necessary part of a liberal and tolerant society that no one should be targeted because of a characteristic with which another takes issue. Your offence was planned. It was deliberate. It was disproportionate. It was dangerous. You have shown no regret."

Oldfield would not be deterred from protesting again, his wife, Deepa Naik, said.

Speaking outside court, the 35-year-old attacked Britain and London as being brutal and deeply divided. She said: "Trenton has spent his adult life working on these issues and his direct action protest on the 7th April 2012 was a natural extension of his everyday work. Trenton's protest was a reaction to an increasingly brutal business, media and political elite."